This invention relates generally to the field of raster correction circuits, and, in particular, to correction of orthogonality and parallelogram errors in a raster of a cathode-ray tube of a video display apparatus.
A deflection system utilized in a video display apparatus, such as a television receiver or a video display monitor, typically includes circuitry that allows for the adjustment of a raster on the viewing screen of the apparatus's cathode-ray tube. Such circuitry is required because of, among other things, the nature of the scanning process and the geometry of the cathode-ray tube.
For example, such circuitry may include a raster correction circuit for eliminating orthogonality and parallelogram errors in the scanned raster. The nature of the orthogonality and parallelogram errors and an approach to eliminating both of them is described in a U.S. patent application entitled "VERTICAL DEFLECTION CIRCUIT WITH RASTER CORRECTION", which was filed on May 17, 1996, in the name of Walter Truskalo et al., and was assigned Ser. No. 08/649,409. That application discloses an arrangement for modulating a vertical deflection current at a horizontal rate for substantially offsetting a downhill scan effect caused by vertical deflection of the electron beam, thereby correcting orthogonality and parallelogram errors in the raster. A raster subject to orthogonality and parallelogram errors is illustrated in FIG. 1.
Such circuitry may also include a centering circuit for, illustratively, horizontally centering the raster on the viewing screen of the tube. Centering the raster is necessary to ensure the most efficient use of the tube, which occurs when the size of the scanned raster is substantially the same size as the tube's viewing screen. The need for horizontal centering is most pronounced when the amount of horizontal overscan is reduced or, in other words, when the size of the scanned raster is reduced to the size of the tube's viewing screen. Centering the raster is typically accomplished by causing a direct current of selected polarity and amplitude to flow through the appropriate deflection coils, either horizontal or vertical.
In the manufacture of a video display apparatus, it is desirable to consolidate circuitry to the greatest possible extent. Such consolidation provides several advantages, among them: decreased parts count, decreased cost, increased reliability, and an increase in the amount of space available within the apparatus's chassis. Accordingly, it is desirable to consolidate the circuits that perform the raster correction and centering functions.